Shortening start-up times and improving starting reliability while increasing the number of starts is one of many new requirements with respect to plant flexibility that has arisen as a result of the increased use of renewable energy sources such as solar and wind.
A major factor limiting the load output of an existing combined cycle power plant is the allowed pressure and temperature transients of the steam turbine and the heat recovery steam generator as well as the waiting time required to establish warm-up times in the balance of plant and the main piping system. These limitations may also influence the start-up capability of the gas turbine of a combined cycle plant by linking the start-up of the gas turbine with the start-up of the steam turbine.
A method of warming a steam turbine involves using main steam generated from the start-up of a gas turbine or auxiliary steam from other sources generated from within the power plant. This pre-warming is required even for small steam turbines in order to avoid differential temperatures between inner and outer walls of the steam chest, and within the rotor. Unless this is done before the unit is exposed to nominal steam system pressures and temperatures, temperature differentials may create excessive stress in the turbine and/or the turbine steam control valve(s).
Larger steam turbines typically include the step of rolling the turbine during pre-warming. If steam is used to pre-warm the turbine, this introduces further constraints on the pre-warming process by restricting the flow rate of the pre-warming medium. For example, if the turbine is being rolled during the pre-warming process, or if the flow rate of the pre-warming medium is too high through the nominal steam path, the turbine may roll-off the turning gear as it accelerates prematurely. However, if the pre-warming medium flow rate is too low, the heat-up time will be unnecessarily extended.
Modern combine cycle power plants with gas and steam turbines typically require pre-warming of the steam turbine prior to start-up due to the absence of bypass stack between gas turbine and Heat Recovery Steam where typically steam is generated to operate the steam turbine as the absence of such a stack requires parallel starting of the gas turbine and the steam turbine. However, starting the steam turbine requires that the steam temperatures are in accordance with the temperatures of the steam turbine rotors.
With grid specifications requiring high flexibility this necessitates, in particular during the start-up phase, high load gradients of the gas turbine without significant delays are required. This makes a simultaneous parallel start of gas turbine and steam turbine extremely difficult. This means the steam turbine can't be started and remain in steam bypass mode. The result is that the gas turbine, in accordance with the grid operating, must be first operated at relatively high load with the consequence of high gas outlet temperatures. As this result, steam is generated in downstream heat recovery system that has too high a temperature to be used to cold or warm start a steam turbine with excess lifetime loss.
There is therefore an ongoing need for improved pre warming methods to provide fast steam turbine start-up in order to meet flexibility requirements imposed by non-continuous energy sources such as renewable energy and well as improving steam turbine start-up in combined cycle power plants.